Detective work solves a genetic mystery

25 July 2003

By Alison Handmer

Sydney PhD candidate Marc Buhler believes he has tracked down the source of an inherited shield against AIDS, an allele of the CCR5 chemokine receptor, which is shared by many people of Jewish and Viking ancestral origins.

Marc Buhler speaking at the International Congress of Genetics in Melbourne
Marc Buhler speaking at the International Congress of Genetics in Melbourne

Mr Buhler made headlines with a talk at the recent International Congress of Genetics which examined why one in every five Caucasians appears to have a common ancestor who carried the allele, a 32-base deletion mutation of the CCR5 gene.

He believes the person with the original mutation lived around 800AD north-east of the Black Sea in the ancient Kingdom of Khazars, where the upper class became Jewish around that time.

The allele was carried to Scandinavia when Swedish Vikings came by river to the area between 800 and 1000 in search of Arab coins. Later, Ashkenazi Jews are thought to have come into contact with the allele when they left Germany after the Black Death in 1350, some travelling east to join their Jewish cousins in Khazaria.

Mr Buhler, who works at the Institute for Immunology and Allergy Research within the Westmead Millennium Institute, has been researching the incidence of the gene in Australia. He genotyped the CCR5-delta32 allele in 807 Australian Ashkenazi Jews and 311 non-Jewish Australians, and found the allele in one in four Ashkenazi Jews, and in one in three with grandparents from Russia, Poland, Austria, or Czechoslovakia.

A previous study had also found about one in three Ashkenazi Jews to have the allele, along with about one in four Icelanders. But how did people from such widely different regions come to share the same genetic inheritance?

Mr Buhler said: "Our early aim was to look at defining an ancestral haplotype, or linked common pattern, for this allele with a set of microsatellite markers.

"These are the sets of repeated units that are very polymorphic, found in various forms scattered all over the genome. They are used in paternity, forensic and various research areas such as genome screens."

A breakthrough came when he read a paper by Professor Gerard Lucotte from Paris suggesting that Vikings may have spread the allele across Europe. "I also recalled that a couple of papers had been published by Professor Charles Poser of Harvard about Vikings and the possible spread of susceptibility to multiple sclerosis," he said.

"One passage, about the Swedish Vikings going east and down the Russian rivers to establish trade with Arabs via the Khazar Kingdom, just clicked the idea into place for me." Mr Buhler plans to finalise his PhD and then further explore the Icelandic population. "There are hints of differences in the decay of the haplotype between non-Jewish Caucasians - those the Vikings may have passed the gene on to - and the Ashkenazi Jewish community," he said.

"Further fine-mapping of the numerous markers the genome projects have defined in the region may also be worth doing."

He is also interested in the influence of smallpox, which is likely to be involved in selection for this allele, plus the influence of HIV and autoimmune disease.

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